Electrophotographic imaging processes and techniques have been extensively described in both the patent and other literature, for example, U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others. Generally, these processes have in common the steps of employing a photoconductive insulating element which is prepared to respond to imagewise exposure with electromagnetic radiation by forming a latent electrostatic charge image. A variety of subsequent operations, now well-known in the art, can then be employed to produce a permanent record of the image.
One type of photoconductive insulating element particularly useful in electrophotography employs a composition containing a photoconductive insulating material and optionally an electrically insulating film-forming resinous binder material. A unitary electrophotographic element incorporating such a composition is generally produced in a multilayer type of structure by coating a layer of the above-described composition onto a support previously overcoated with a layer of a conducting material. Alternatively, the above-described composition can be coated directly onto a conductive support of metal or other suitable conductive material.
Typically, it is desirable to improve the speed and/or spectral response of photoconductive compositions generally employed in electrophotographic processes by incorporating in such compositions various sensitizer materials and addenda. Among the various sensitizer addenda which have been found especially effective for use in photoconductive compositions are the dyes selected from the group consisting of pyrylium, selenapyrylium, and thiapyrylium dye salts such as are disclosed in U.S. Pat. Nos. 3,250,615; 3,141,770; 3,679,408; and 3,615,418. Generally, the aforementioned pyrylium, thiapyrylium, and selenapyrylium dye salts heretofor used in photoconductive compositions have been found effective to change the sensitivity or electrical speed of a particular photoconductive composition. Although the mechanism of such sensitization is presently not fully understood, the phenomenon has been found extremely useful.
The importance of such effects is evidenced by the extensive search currently conducted by workers in the art for compositions and compounds which are capable of photosensitizing photoconductive compositions in the manner described.
Usually the desirability of a change in electrophotographic properties is dictated by the end use contemplated for the photoconductive element. For example, in document copying applications the spectral electrophotographic response of the photoconductor should be capable of reproducing the wide range of colors which is normally encountered in such use. If the response of the photoconductor falls short of these design criteria, it is highly desirable if the spectral response of the composition can be altered by the addition of photosensitizing addenda to the composition. Likewise, various applications specifically require other characteristics such as the ability of the element to accept a high surface potential, and exhibit a low dark decay of electrical charge. It is also desirable for the photoconductive element to exhibit high speed as meansured in an electrical speed or characteristic curve, a low residual potential after exposure and resistance to fatigue. Sensitization of many photoconductive compositions by the addition of certain dyes selected from the large number of dyes presently known has hitherto been widely used to provide for the desired flexibility in the design of photoconductive elements in particular photoconductor-containing systems. Conventional dye addenda to photoconductor compositions have generally shown only a limited capability for overall improvement in the totality of electrophotographic properties which cooperate to produce a useful electrophotographic element or structure. The art is still searching for improvements in shoulder and toe speeds, improved solid area reproduction characteristics, rapid recovery and useful electrophotographic speed from either positive or negative electrostatic charging.
A high speed "heterogeneous" or "aggregate" photoconductive system has been developed which overcomes many of the problems of the prior art. This aggregate composition and certain components thereof are the subject matter of William A. Light, U.S. Pat. No. 3,615,414 issued Oct. 26, 1971 and Gramza et al. U.S. Pat. No. 3,732,180 issued May 8, 1973. The addenda disclosed therein are responsible for the exhibition of desirable electrophotographic properties in photoconductive elements prepared therewith. However, use of the pyrylium type sensitizing dyes described therein quite often results in an aggregate photoconductive system which exhibits an absorption minimum (and therefore a somewhat lower sensitivity than would be desired) to light in some portion of the visible spectrum, for example, in the blue region of the spectrum, i.e. light having a wavelength within the range of from about 400 to about 500 nm. Accordingly, there is a need in the art for means of obtaining high speed aggregate photoconductive compositions which can be selectively modified to provide improved spectral response in those regions of the electromagnetic spectrum where the aggregate composition exhibits somewhat lower absorption than is desired.